In mobile communication networks, such as LTE (Long Term evolution), random access channels are used by mobile communication terminals to start communication with a base station using a cell of the base station. The mobile communication terminal initiates this by sending a preamble, consisting of a number of symbols, on the random access channel (which is common to all such initiation for the cell) with a preamble id (RAPID) between 0 and 63 selected from a group of preambles which are valid for the cell of the base station. In LTE, for the same cell, the valid preambles use a finite set of symbol sequences (root sequences) but different preambles using the same symbol sequence are cyclically shifted from each other by a number of symbols.
A delay occurs for the transmission of the preamble from the mobile communication terminal to the base station. This delay can be measured in the number symbols of the sequence. If, due to a distance between the mobile communication terminal and the base station, the propagation delay is larger than a shift between two valid preambles, the base station can interpret the preamble with the wrong preamble id. In this case, the set up procedure fails, whereby the mobile communication terminal needs to try again with the same or different base station cell. This can lead to longer set up times, failed handovers, etc.
It is thus desired to have a large random access cell size to allow for longer delays and fewer failed set up procedures. On the other hand, it is desired to have a small random access cell size to prevent preamble confusion between neighbouring random access cells. By using small cells, more preambles are available in the system with reduced risk of the same preamble being used in neighbouring cells. It is thus difficult to find the right balance to determine an acceptable cell size.
Previously, this cell size has been set during network planning.